1. Field of the Invention
The present invention relates to a focus detection system for image pickup apparatuses such as cameras, video cameras, etc., or for observation apparatuses of various kinds, and more particularly to a focus detecting device capable of detecting focus two-dimensionally and continuously over a wide range on a photo-taking image plane or observation image plane.
2. Description of Related Art
FIG. 8 shows a conventional camera having a built-in focus detecting device. The camera shown in FIG. 8 includes an objective lens 101 which is provided for photo-taking, a main mirror 102 which is semi-transparent, a focusing screen 103, a pentagonal prism 104, an eyepiece 105, a sub-mirror 106, a film 107, and a focus detecting device 108.
Referring to FIG. 8, a light flux from an object (not shown), after passing through the objective lens 101, is reflected upward by the main mirror 102 to form an image on the focusing screen 103. The image formed on the focusing screen 103 is reflected a plurality of times by the pentagonal prism 104 and is then viewed through the eyepiece 105 by a camera operator or an observer.
A part of the light flux which reaches the main mirror 102 from the objective lens 101 passes through the semi-transparent main mirror 102 and is then reflected downward by the sub-mirror 106 to be guided to the focus detecting device 108.
FIG. 9 is a diagram for explaining the operating principle of the focus detecting device by showing, in development view, essential parts of the objective lens 101 and the focus detecting device 108 shown in FIG. 8.
Referring to FIG. 9, the focus detecting device 108 includes a field mask 109 which is disposed near to a predetermined focal plane of the objective lens 101, i.e., a plane conjugate to a film surface, a field lens 110 which is disposed also near to the predetermined focal plane, a secondary image forming system 111 which is composed of two lenses 111-1 and 111-2, a photoelectric conversion element 112 which is composed of two sensor arrays 112-1 and 112-2 disposed respectively behind the two lenses 111-1 and 111-2, and a diaphragm 113 which has two aperture parts 113-1 and 113-2 disposed respectively correspondingly with the two lenses 111-1 and 111-2. Reference numeral 114 denotes an exit pupil of the objective lens 101 including two divided areas 114-1 and 114-2.
The field lens 110 is arranged to form images of the aperture parts 113-1 and 113-2 respectively in the neighborhood of the areas 114-1 and 114-2 of the exit pupil 114 of the objective lens 101. The quantities of light of light fluxes 115-1 and 115-2 having passed through the areas 114-1 and 114-2 are thus distributed respectively to the two sensor arrays 112-1 and 112-2.
The focus detecting principle of the focus detecting device shown in FIG. 9 is generally called a phase-difference detecting method. According to this method, the light-quantity distributions respectively obtained on the two sensor arrays 112-1 and 112-2 come near to each other when the image forming point of the objective lens 101 is in front of the predetermined focal plane, i.e., on the side of the objective lens 101, and come away from each other when the image forming point of the objective lens 101 is in rear of the predetermined focal plane, i.e., on the side opposite to the objective lens 101. Besides, the amount of discrepancy between the light-quantity distributions obtained on the two sensor arrays 112-1 and 112-2 is in a functional relation to the amount of defocus, i.e., the amount of focus deviation, of the objective lens 101. Therefore, the amount of defocus and the direction of defocus of the objective lens 101 can be detected by computing the amount of discrepancy with a suitable computing means.
The focus detecting device shown in FIG. 9 is capable of detecting focus only for an object in one area located in the central part of an observing range or a photo-taking range of the objective lens 101. In view of this, a focus detecting device has been developed to be capable of detecting focus not only for the central area but also for an object located outside of the central area of the observing or photo-taking range.
FIG. 10 shows the arrangement of an optical system of the above-stated focus detecting device. In FIG. 10, reference numeral 116 denotes a field mask. The filed mask 116 has a cross-shaped aperture part 116-1 formed in the middle part thereof and vertical oblong aperture parts 116-2 and 116-3 formed in its peripheral part on both sides of the cross-shaped aperture part 116-1.
A field lens 117 is composed of three parts (areas) 117-1, 117-2 and 117-3 which correspond respectively to the three aperture parts 116-1, 116-2 and 116-3. A diaphragm 118 is provided with a middle aperture part 118-1 and peripheral aperture parts 118-2 and 118-3. The middle aperture part 118-1 includes four apertures 118-1a, 118-1b, 118-1c and 118-1d which are arranged in vertical and transverse pairs. The peripheral aperture part 118-2 includes a pair of apertures 118-2a and 118-2b and the peripheral aperture part 118-3 includes a pair of apertures 118-3a and 118-3b. The areas 117-1, 117-2 and 117-3 of the field lens 117 are arranged respectively to form images of the aperture parts 118-1, 118-2 and 118-3 in the neighborhood of the exit pupil of an objective lens (not shown). An optical member 119 is a secondary image forming system which is integrally formed with four pairs of lenses, i.e., eight lenses, 119-1a, 119-1b, 119-1c, 119-1d, 119-2a, 119-2b, 119-3a and 119-3b, disposed respectively in rear of the corresponding apertures of the diaphragm 118. A photoelectric conversion element 120 is composed of four pairs of, i.e., a total of eight, sensor arrays 120-1a, 120-1b, 120-1c, 120-1d, 120-2a, 120-2b, 120-3a and 120-3b, which are arranged to receive images from the corresponding lenses of the secondary image forming system.
FIG. 11 shows the manner of images which are formed on the photoelectric conversion element 120. Referring to FIG. 11, light fluxes having passed through the middle aperture part 116-1 of the field mask 116 and the middle part 117-a of the field lens 117 are respectively restricted by the apertures 118-1a, 118-1b, 118-1c and 118-1d of the diaphragm 118 and, after that, are respectively imaged on image areas 121-1a, 121-1b, 121-1c and 121-1d of the photoelectric conversion element 120 by the lenses 119-1a, 119-1b, 119-1c and 119-1d of the secondary image forming system 119 disposed behind the diaphragm 118. Light fluxes having passed through the peripheral aperture part 116-2 of the field mask 116 and the peripheral part 117-2 of the field lens 117 are restricted by the apertures 118-2a and 118-2b of the diaphragm 118 and, after that, are imaged on image areas 121-2a and 121-2b of the photoelectric conversion element 120 by the lenses 119-2a and 119-2b of the secondary image forming system 119 disposed behind the diaphragm 118. Light fluxes having passed through the peripheral aperture part 116-3 of the field mask 116 and the peripheral part 117-3 of the field lens 117 are likewise restricted by the apertures 118-3a and 118-3b of the diaphragm 118 and, after that, are imaged on image areas 121-3a and 121-3b of the photoelectric conversion element 120 by the lenses 119-3a and 119-3b of the secondary image forming system 119 disposed behind the diaphragm 118.
The focus detecting principle of the focus detecting device shown in FIG. 10 is similar to that shown in FIG. 9. Focus is detected by detecting the relative positions of images obtained in the direction of arrays of paired sensors. According to the arrangement shown in FIG. 10, focus can be detected not only for an object located in the central area of the observing or photo-taking range but also for objects located in positions corresponding to the peripheral aperture parts 116-2 and 116-3 of the field mask 116. Further, the above-stated arrangement enables the focus detecting device to detect focus even when a light-quantity distribution of a photo-taking or observing object varies only in one vertical or lateral direction in the central area of the observing or photo-taking range.
With each of the above-stated focus detecting devices used for a camera having an interchangeable lens, such as a single-lens reflex camera, it is sometimes impossible to correctly detect a focusing state, if the lens is controlled on the basis of a focusing-state detecting signal related to an amount of focus deviation directly obtained. A main reason for this problem lies in that a light flux of the objective lens forming an observing or photo-taking image generally differs from a light flux taken in by the focus detecting device.
Another reason lies in that the focus detecting device of the phase difference detecting type is arranged to detect a focus position, i.e., a focus deviation amount, by converting it into an image discrepancy with respect to a lateral aberration, while the focus deviation amount should be determined with respect to the amount of longitudinal aberration, i.e., an aberration in the direction of an optical axis. Therefore, in a case where the objective lens has some aberration, there arises a difference between the light flux of the objective lens and the light flux taken in by the focus detecting device according to how the aberration is corrected.
To solve these problems, a lens control method has been developed to carry out lens control on the basis of a corrected focus detection signal Dc obtained by some correction means. The correction means is arranged to correct a focus detecting signal D indicative of the amount of focus deviation by using a correction value C decided for each individual objective lens, so as to obtain the corrected focus detection signal Dc, for example, as expressed below: EQU Dc=D-C (1).
The correction value C for each individual lens generally varies according to the position of a focus detecting area of the focus detecting device. Therefore, in a case where there are focus detecting areas besides the central focus detecting area as shown in FIG. 10, the focus detecting device must be provided with correction values for all of these focus detecting areas. However, in a case where use of many focus detecting areas is anticipated, the above-stated lens control method necessitates a large storage capacity either on the side of the objective lens or on the side of the camera body for storing many correction values for all the focus detecting areas.
In cases where the aberration of an objective lens greatly varies as a result of a change of the position (a focusing object distance) of a focusing lens within the objective lens, or a change of a focal length of a zoom lens or a change of aperture of a diaphragm in taking a shot, many correction values must be kept in store to cover the moving states of the lens for focusing or zooming and the aperture positions of the diaphragm. Such requirements necessitate a further increase in storage capacity. The increase in storage capacity can be suppressed to some degree by limiting the number of divisions of correction values for each of such states. However, that method is undesirable because the storage capacity is suppressed at the expense of precision of the correction.
Further, in the case of a photo-taking system already arranged to act only for predetermined positions or a predetermined number of focus detecting areas, such as a single-lens reflex camera, an interchangeable lens or the like, a new camera having a focus detecting device arranged to have different positions and a different number of focus detecting areas does not adequately operate in that system.
A method for solving these problems was disclosed in Japanese Laid-Open Patent Application No. HEI 6-331886. According to this method, the correction value C is assumed to be dependent only on a distance e from the center of the focus detecting area and a change in the correction value C is assumed to be expressible by a function related to the distance l. Correction values for at least two focus detecting areas in specific positions are used as they are. The correction values for focus detecting areas in other positions are, on the other hand, obtained through a correcting process carried out with a function of a linear or quadratic expression.